The present invention relates to dead-end anchorages. More particularly, the present invention relates to methods and apparatus which are used so as to mechanically secure the end of a tendon within an interior cavity of an anchor. The present invention also relates to dead-end anchorage forming mechanisms in which a compressive force is applied to the end of the tendon.
For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, concrete design began to evolve. Concrete has the advantages of costing less than steel, of not requiring fireproofing, and of having plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, comprising a mixture of water, cement, sand, and stone or aggregate and having proportions calculated to produce the required strength, is set, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as five hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
In typical post-tension systems, the tendon is received between a pair of anchors. One of the anchors is known as the xe2x80x9clive endxe2x80x9d anchor, and the opposite end is known as the xe2x80x9cdead-endxe2x80x9d anchor. The xe2x80x9clive endxe2x80x9d anchor receives the end of the tendon which is to be tensioned. The xe2x80x9cdead-endxe2x80x9d anchor holds the tendon in place during the tensioning operation. Under typical operations, a plurality of wedges are inserted into an interior passageway of the anchor and around the exterior surface of the tendon. The tendon is then tensioned so as to draw the wedges inwardly into the interior passageway so as establish compressive and locking contact with an exterior surface of the tendon. This dead-end anchor can then be shipped, along with the tendon, for use at the job site.
One technique for forming such dead-end anchors is to insert the end of a tendon into the cavity of the anchor, inserting wedges into the space between the tendon and the wall of the cavity and then applying a tension force onto another end of the tendon so as to draw the wedges and the end of the tendon into the cavity in interference-fit relationship therewith. This procedure is somewhat difficult since the tendon can have a considerable length and since the use of tension forces can create a somewhat unreliable connection between the wedges and the tendon. Experimentation has found that the application of compressive force onto the end of the tendon creates a better interference-fit relationship between the wedges, the end of the tendon and the wall of the cavity of the anchor.
FIG. 1 shows one such type of compression system for the forming of a dead-end anchor. In FIG. 1, it can be seen that a fixture 10 is provided on a base 12 having a channel 14 suitable for receiving a tendon 16 in a desired position. An anchor 18 is connected to the tendon 16 and resides against a wall 20 of the fixture 10. In this arrangement, the wide end of the cavity of the anchor 18 faces outwardly. A compression mechanism 22 has a plunger 24 at one end. The compression mechanism 22 includes a hydraulic or pneumatic system 26 for the purpose of applying strong pressures to the plunger 24. The plunger 24 includes an indentation 28 at the end 30 so as to allow the end of the tendon 16 to be inserted therein. When suitable hydraulic pressure is applied to the plunger 24, the plunger 24 will move toward the anchor 18 so as to apply compressive forces onto the end of the tendon 16 for the purpose of establishing a strong interference-fit relationship between the tendon, the wedges and the wall of the cavity of the anchor 18.
FIG. 2 is a more detailed view of the prior art system of FIG. 1 showing the formation of the dead-end anchorage. In particular, in FIG. 2, there is shown the anchor 18 as having a steel anchor body 32 with a polymeric encapsulation 34 extending therearound. The anchor body 32 includes an interior cavity 36 which tapers inwardly from end 38 toward end 40. Wedges 42 are positioned in the cavity 36 between the exterior of the tendon 16 and the inner wall 44 of the cavity 36. The plunger 24 is shown as having indentation 28 at the end 30. The plunger 24 will move toward the end 38 of the anchor 18 so as to force the tendon 16 and the wedges 42 into the cavity 36.
In the normal process of using the system of FIGS. 1 and 2, the anchor 18 is initially installed within its desired position in the fixture 12 so that a surface of the anchor 18 abuts the wall 20 of the fixture 10. The end of the tendon 16 is positioned within the cavity 36 of the anchor 18 so as to have its end 46 extending outwardly of the end 38 of the anchor 18. The wedges 42 are then installed, by hand, into the cavity 36 in the gap between the wall 44 of the anchor 18 and the exterior surface of the tendon 16. As shown in FIG. 2, these wedges 42 can only be installed by hand a small distance into the cavity 36. The plunger 24 is then activated so that the end 46 of the tendon 16 will enter the indentation 28 of the plunger 24. The strong forces imparted by the compression mechanism 26 force the tendon 16 and the wedges 42 into the cavity 36. The plunger 24 is then retracted so that the anchor 18 can be removed from the fixture 10.
There are several problems with the system shown in FIGS. 1 and 2. First, the wedges 42 must be positioned by hand. In certain circumstances, the wedges 42 may be positioned unevenly. In other circumstances, the use of such strong hydraulic mechanisms can cause injury if the user""s hands are not removed from the area between the anchor 18 and the end 28 of the plunger 24. The indentation 28 has a relatively large size compared to the end 46 of the tendon 16. This wide opening is required in case a burr or other deformation has occurred in the end 46 of the tendon 16. Since the indentation 28 is relatively large, the end 46 of the tendon 16 can have a tendency to buckle when the forces are applied by the plunger 24. Furthermore, the mechanism shown in FIGS. 1 and 2 makes it very difficult to control the distance between the end of the wedges 42 and the end 46 of the strand 16.
It is an object of the present invention to provide a method and apparatus for forming a dead-end anchorage which installs the tendon in the anchor by compression forces.
It is another object of the present invention to provide a method and apparatus which eliminates the hand positioning of wedges during such compression forming.
It is another object of the present invention to provide a method and apparatus which eliminates any buckling of the end of the strand during compression fitting.
It is still another object of the present invention to provide a method and apparatus which properly meters the distance between the end of the strand and the end of the wedges.
It is still a further object of the present invention to provide a method and apparatus for forming a dead-end anchorage which is safe, easy to use, and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.
The present invention is a method for forming an anchorage of a post-tension system comprising the steps of: (1) positioning a tendon within a cavity of an anchor body such that an end of the tendon extends outwardly of the cavity; (2) mechanically inserting wedges within the cavity between the tendon and a wall of the cavity; and (3) applying pressure onto the end of the tendon such that the tendon and the wedges are in interference-fit relationship within the cavity.
In the method of the present invention, a compression mechanism is used which has a cylindrical member and a plunger extending in a channel interior of the cylindrical member. In the method of the present invention, the wedges are attached to the cylindrical member, and then the cylindrical member is moved toward the cavity such that the wedges enter a space between the tendon and the wall of the cavity. Specifically, in a preferred embodiment of the present invention, the wedges are magnetically attached to an end of the cylindrical member. The cylindrical member is moved toward the cavity such that the end of the tendon resides within the interior space of the cylindrical member. The plunger is moved through the interior channel toward the end of the tendon such that the end of the plunger applies pressure onto the end of the tendon within the space. The anchor can be affixed within a fixture such that a wide end of the cavity faces the wedges. In the method of the present invention, the cylindrical member and the plunger can be retracted from the end of the tendon after the proper pressure is applied to the end of the tendon.
The present invention is also an apparatus for forming the anchorage of a post-tension system comprising a housing, a piston member positioned within the housing and movable relative to the housing, a cylindrical member connected to the piston member and having a channel formed therein, and a plunger axially movable within the channel in the cylindrical member. A frame receives the housing therein and has a portion extending outwardly of an end of the cylindrical member. A fixture is provided which is suitable for receiving an anchor therein. A plurality of wedges are releasably secured to an end of the cylindrical member. The cylindrical member is resiliently mounted within the piston member. The cylindrical member has an end extending outwardly of an end of the piston member. The plunger is connected to the piston so as to move through the channel within the cylindrical member when a resistive force is applied to an end of the cylindrical member. The anchor is positioned so as to have a wide end of a cavity facing the cylindrical member. The tendon extends through the cavity so as to have an end extending outwardly thereof. The channel of the cylindrical member has a size suitable for receiving the end of the tendon therein.